Named after a county in California, the Yolo was invented by Arthur S. Leonard of the University of California, Davis. It differs from a Schiefspiegler in that both mirrors are concave. The Yolo enjoys three advantages over the Schief with, (1) lower focal ratios possible, (2) lower image tilts, and (3) a more compact form. Its one major drawback probably accounts for so few Yolos built; it cannot correct for astigmatism without help. Three methods to do this have been successfully employed. Briefly, they are (1) the use of a warping harness, (2) polishing the correction into the secondary, and (3) adding a spectacle lens inside focus.

	From a design by José Sasian in "Telescope Making" #37, Albert Priselac built this 6" f/10.2 Yolo.
	Erwin Herrig of the former East Germany, without access to western literature, re-invented the Yolo.
	Close-up view of Erwin Herrig's harness for warping the Yolo secondary prior to final grinding and polishing. When the harness is released, the finished mirror assumes a toroidal figure.
	The covering has been removed from the telescope here to show internal details. An English translation of Erwin Herrig's method of figuring his toroidal secondaries is available.